Cooling system



Feb. 25 1958 w. E. BIRCHARD 2,82

COOLING SYSTEM Filed April 12, 1956 Unite States Patent COOLING SYSTEM Wayne E. Birchard, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application April 12, 1956, Serial No. 577,750 6 Claims. (Cl. 336-61) This invention relates to a cooling system, and more particularly to a cooling system for heat generating apparatus such as stationary electrical induction apparatus, for example, electrical transformers.

In the electrical industry forward steps are continuously being made to reduce the size and weight of transformers. Reductions in the size and weight of transformer components such as core iron, winding copper and insulating liquid mean that the transformer will have a reduced inherent or natural thermal capacity or heat storage ability. That is, a reduction in the mass of the transformer reduces its ability to absorb large quantities of heat without an appreciable temperature rise. Such reduction in thermal capacity is a disadvantage during overload periods of short duration. The additional heat generated during overload conditions may cause a temperature rise in the apparatus beyond its safe maximum or maximum operating desired temperature if the thermal capacity of the apparatus is not large enough to absorb this additional heat without a sharp temperature rise. It will be appreciated that operation of transformers at high temperature overloads materially reduces the life of the transformer by materially reducing the useful life and strength of the electrical insulation.

It is an object of this invention to provide an improved method and means for limiting the temperature rise of heat generating apparatus or heated components.

It is another object of this invention to provide a method and means for increasing the overload capacity of transformers.

In my invention the temperature rise of heat generating apparatus or heated components is limited and the overload capacity of transformers is increased by positioning .a high latent heat of fusion material in thermal absorbing relationship with said heat generating apparatus or heated components and transformers respectively.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a broken away perspective view of one form of my invention; and

Fig. 2 is a broken away enlargedperspective view of a part of the apparatus of Fig. l; and

Fig. 3 is a broken away perspective view of another form of my invention; and

Fig. 4 is a broken away perspective view of still another form of my invention.

Referring now particularly to Fig. 1, illustrated there in is a stationary electrical induction apparatus or transformer comprising a magnetic core 1 and three electrical winding cylinders 2 which are linked with theicore 1. Thecore 1 is held in assembled position'by core clamps 3,1and each of the winding cylinders 2 has low voltage an'dhigh voltage winding sections. The apparatus is enclosed within a sealed tank 4 which has a plurality of low voltage bushings 5 and high voltage bushings 6 mounted on the cover. The apparatus is a well known form of a three phase transformer and the various parts thereof will not be described in detail inasmuch as it is believed that this is unnecessary for a complete understanding of my invention.

The tank 4 may be rectangular and may be liquid or gas-filled, and positioned in each corner of the tank 4 is a closed container 7 of high latent heat of fusion material. The containers 7 are better illustrated in Fig. 2 and the high latent heat of fusion material is indicated by reference numeral 8. By a high latent heat of fusion material is meant a substance which will absorb a large quantity of heat in changing from a solid to a liquid state as compared to the specific heat of the apparatus or components being cooled such as the core 1, windings 2, and the fluid in which the apparatus or components are immersed. The containers 7 may have a triangular cross section so that they will fit snugly into the four corners of the tank 4. Also, containers 7 may have a plurality of fins 9 formed thereon to aid in the absorption of heat by the material 8. The container 7 may be mounted inside the tank 4 by welding the side walls of the container 7 to the side walls of tank 4. A mounting arrangement in which the side walls of container 7 are in direct contact with the side walls of tank 4- is desirable inasmuch as this relationship facilitates the dissipation of heat from material 8 to the ambient air surrounding tank 4. Provision is made for the expansion and contraction of material 8 by not completely filling the container 7.

During short overload periods of several hours duration the material 8 will limit the temperature rise of the transformer by absorbing a large quantity of heat in changing from a solid to a liquid state. For this to be accomplished several factors must be considered. For instance, the latent heat of fusion of material 8 must be high enough so that it will absorb a large enough quantity of heat to be effective to limit the temperature rise at overloads. Also, in order to save on space the latent heat of fusion should be high enough so that a relatively small quantity of material will do the job. Additionally, the melting point of material 8 must be properly selected if it is to be effective to increase the overload capacity of the apparatus. For instance, if substantially all of material 8 is positioned in the top of the tank 4 then it should have a melting point temperature which approximates the desired maximum top oil or gas operating temperature. If a rapid response in absorbing heat by material 8 is desired then the melting point may be as much as several degrees below the maximum desired'top oil or gas operating temperature. However, since material 8 is a reserve heat absorber the melting point should not be too far below the maximum desired top oil or temperature. Otherwise material 3 will have been substantially completely melted before overload conditions are reached. if a less rapid response by material 8 in absorbing heat is desired then it can have a melting point which is the same as or more nearly equal to the desired maximum top oil or gas operating temperature. Also, the melting point selected preferably should not be above the desired maximum top oil or gas operating temperature lest the ability of material 8 to absorb a large block of heat not come into play until after the maximum desired operating temperature has been exceeded.

'5 he melting point temperature selected will also depend:

instance, when substantially all of material 2'5 is positioned" within the top oil-or gas its meltiug'pcint will need to be higher than when it is positioned further down in the oil or gas where operating temperatures are lower.

This thermal process of changing material 8 from a solid to a liquid state should be completely reversible. That is, after the load on the apparatus is reduced the heat absorbed by material 3 should be freely dissipated to the ambient and the material 8 should revert to its solid state whereby it will be in reserve to again add to the inherent thermal capacity of the apparatus during a later overload period.

Generally speaking it is not believed that the invention would be very useful in apparatus which operates continuously at constant load under constant ambient conditions. Rather, the invention is particularly useful in apparatus which has repetitive thermal cycles and where it is desired to limit the temperature rise during short periods of high heat generation in relation to the overall thermal cycle time. For example, some transformers have'daily thermal cycles. During some portion of each day the transformer is carrying low loads, as during the early morning hours. During another portion of the day the transformer may be carrying moderate loads, as during daylight hours. Still another portion of the day, as during the evening hours, is usually a period of peak loads, and it is during this period that overloads may be most likely encountered.

In such a daily thermal cycle pattern my invention can be used during the peak load period to limit the temperature rise of the apparatus due to overloads. If the high latent heat of fusion material gains heat during such time it will be understood that it will have cooled and reverted back to its solid state during the subsequent period of low load so that it will be in condition to limit the temperature rise during a subsequent overload. Thus, my invention is intended to be used in thermal cycles which have operating temperatures during some part thereof which are sufficiently lower than the melting point of the high latent heat of fusion material so that there will be some time within the overall thermal cycle for the material to revert to its solid state. Understandably if the material is losing heat during the early morning hours this heat dissipation may raise the operating temperature of the transformer somewhat during this period. However, this operating temperature must be less than the melting point of the material if the material is to change from a liquid to a solid by giving up its stored heat.

Heretofore it was stated that the invention is intended to be used to limit the temperature rise during overloads of short time duration in reference to the total time duration of the thermal cycle. For instance, in a transformer application the overload period may be of several hours duration, but this period is short in relation to the 24 hours of the total thermal cycle.

One material which may be used in practicing the invention is naphthalene which has a melting point temperature of about 80 C. About 90 pounds or 1 /2 cubic feet of this material was placed in the top oil of a conventional 50 kva. oil filled pole type transformer and the transformer was operated at 50 percent of rated load until steady state temperature conditions were reached which was below the naphthalene melting point temperature. Then the load was increased to 175 percent of rated load and this increased load was maintained until all the naphthalene was melted at which time the effective oil temperature was about 95 C. Effective oil temperature is a value between top and bottom oil temperature. It took about 8% hours from the 50 percent load steady state condition to melt the naphthalene. This procedure was repeated without the naphthalene and the time required under 175 percent load from 50 percent load steady state conditions to attain an effective oil temperature of 95 C. was about hours. In other words, the invention increased the period of overload required to produce an effective oil temperature of C. by about 70 percent in another set of tests the transformer was operated at percent rated load until steady state temperature conditions were reached and then the load was increased to 200 percent. At 200 percent load it took about 3% hours to melt all of the naphthalene at the end of which time the effective oil temperature was about 100 C. Without the naphthalene it took 2% hours of 200 percent load to attain an effective oil temperature of about 100 C. Therefore, in this second set of tests the invention increased the period of overload required to produce an effective oil temperature of 100 C. by about 47 percent in all tests ambient conditions were about the same.

In view of the illustrated use of my invention it will be apparent that the question of what is an overload is dependent in large part upon the use to which my invention is put. For instance, in transformers what is an overload for purposes of utilizing the invention de pends upon what one wants to accomplish. A maximum safe temperature can he arrived at, as determined in part by the electrical insulation, beyond which it would be unwise to operate the transformer. However, it does not necessarily follow that all loads which give operating temperatures less than this maximum safe temperature are not overloads. The expected life of a transformer may be figured on the assumption that during the relatively larger portion of its expected life it will be operated at temperatures considerably less than this maximum safe temperature and that the transformer will be operated at temperatures very close to this maximum safe temperature only for a short portion of its expected life. However, some utilities may have a small number of transformers in a given system and operate them as much as possible at loads that give operating temperatures close to this maximum safe temperature even though this will reduce the actual life of the transformers to less than their expected life under more usual operating conditions. This is done on the theory that it is more economical to use fewer transformers even though they must be replaced sooner rather than use a larger number of transformers in the same system even though the latter may givefull life expectancy, it being understood that the more a transformer is operated close to this maximum safe temperature the shorter its actual life will be. Those utilities which prefer the smaller number transformer system arrangement may consider only those loads which give temperatures greater than this maximum safe temperature as being undesirable overloads which are to be guarded against by my invention. However, those utilities which prefer to get full life expectancy may consider loads which give temperatures closely approaching this maximum safe temperature as overloads to be guarded against. Therefore, what is a desired maximum operating temperature will not be the same for all users of my invention. That is, for one user it may be a temperature which will be exceeded relatively infrequently in emergency situations and beyond which it is unwise to go, and for another user it may be a considerably lower temperature, and for still another user it may be a point between these two temperatures.

In the Fig. 3 form of my invention the containers 7' of high latent heat of fusion material are placed in direct contact with the core clamps 3 of the core 1. Since the containers 7' are elongated and rectangularly cross-sectioned they will conveniently fit into the channel-shaped core clamps 3 for broad surface contact therewith. This form of my invention is useful in transformers which are vented to the atmosphere. Venting to the atmosphere may be accomplished by opening the bottom and top of the tank to permit a natural air draft therethrough. In the Fig. 1 form of invention inasmuch as the tank 4 is sealed the arrangement provides a closed convection cooling circuit. However, in the Fig. 3 form of my invention inasmuch as the convection circuit is open it is necessary to place the latent heat of fusion material in direct contact with the core structure and in close proximity to the windings 2 so that it will have an appreciable effect on controlling the temperature rise of the apparatus during overload. However, the arrangement of Fig. 3 is not necessarily restricted to vented transformers but can also be employed in sealed liquid or gas-filled apparatuses such as illustrated in Fig. 1.

In the Fig. 4 form of my invention the container 7" of high latent heat of fusion material is placed between one or more of the legs of core 1 and the low voltage winding section 2 although it may also be placed between the low and high voltage winding sections 2 and 2 respectively. The Fig. 4 arrangement like the Fig. 3 arrangement is useful in vented transformers although not necessarily restricted thereto but also useful in liquid or gas-filled transformers. The container 7" of high latent heat of fusion material 8' may comprise a double walled cylindrical member. All of the containers 7, 7' and 7" are preferably constructed from metallic material having a high thermal conductivity. If the cylinder 7" is constructed from metallic material then it can have a slot 9 formed throughout the total length thereof so as to limiteddy currents.

In the formof the invention illustrated in Fig. 1 the melting point temperature of the high latent heat of fusion material 8 was related to the desired maximum top oil or gas operatingtemperature in order to indirectly control the temperature rise ofthe Windings 2 at overload. For any particular desired maximum operating temperature'of the insulated windings there will be a corresponding desired maximum top oil or gas operating temperature. This is so since the insulating fluid is heated by the windings. The insulating fluid is also heated by the core, but for conventional designs is heated more by the windings than by the core during overload. The high latent heat of fusion material will limit the actual top insulating fluid operating temperature to its desired maximum operating value by absorbing heat therefrom during overload. The extraction of heat from the insulating fluid will enable it to pick up additional heat from the windings whereby the actual winding temperature will be limited to its desired maximum value. f course, cooling of the insulating fluid will also result in cooling of the core 1 so the core in turn will be able to pick up heat from the windings during overload.

In a similar manner in Fig. 3 one component of the apparatus is directly cooled in order to control the temperature of another component to the latters desired maximum operating temperature. In Fig. 3 the high latent heat of fusion material will pick up a large quantity of heat from the core structure so the latter in turn can absorb a large quantity of heat from the windings, it being understood that the core structure in part is heated by the windings. Of course, the material will also pick up some heat rather directly from the windings due to its close proximity thereto. However, it will be appreciated that in Fig. 3 the melting point of the material will be related in large part to the desired maximum core temperature which corresponds to the desired maximum temperature of the windings.

In Fig. 4 the high latent heat of fusion material 8 picks up heat directly from both the core 1 and winding 2. Therefore, its melting point temperature will need to be related to that maximum temperature which would exist in the vicinity of the cylinder 7" for the corresponding desired maximum winding temperature. However, the invention is not restricted to arrangements wherein the primary component whose temperature it is desired to controlis indirectly cooled by first cooling another component which in turn will cool the primary component. The invention can also be used to directly cool the component which it is desired to limit to some desired maximum temperature during short time overloads. For instance, if the cylinder '7 were positioned between the winding sections 2' and 2 the high latent heat of fusion material 8 would absorb heat from the windings 2 more directly than in the illustrated arrangements of Figs. 1, 3 and 4. That is, the melting point temperature would be more directly and closely related to the desired maximum temperature of the windings than to possibly any other component. However, it will be appreciated that in transformers it is usual to form axial cooling ducts in the windings in the manner shown in Fig. 4. For this reason heat from the windings tends to rise to a greater degree lengthwise of the windings or ducts rather than crosswise. Therefore, even if the cylinder 7" were positioned between winding sections 2' and 2" the melting point of the material 8' may not be perfectly directly related to the temperature of the windings by selecting its melting point to be just below or equal to the desired maximum copper temperature of the windings.

While there have been shown and described particular embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and therefore, it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric transformercomponent which has a thermal cycle which includes a period of relatively low temperature and may include a period of relatively high temperature, saidhigh temperature period being of relatively short time duration as compared to the total time duration of said cycle, and said component-being positioned inside a main tank, means for limiting the temperature rise of said component during said short time period to a desired maximum temperature, said means comprising a predetermined mass of material having a high latent heat of fusion, said heat of fusion being relatively large as compared to the specific heat of said component, said material being disposed in a container which is positioned inside said tank and in thermal absorbing relationship with respect to said component, said material having a melting point temperature which is below said maximum temperature but above said low temperature and said material being thermally reversible whereby said material will change from a solid to a liquid state during said short time period and then revert to said solid state during said low temperature period.

2. In an electric transformer apparatus which generates heat, said apparatus having a thermal cycle which includes a period of relatively low temperature range and which may include a period of relatively high temperature range, said high temperature range period being of elatively short time duration as compared to the total time duration of said cycle, and said apparatus being housed in a main tank, means for limiting the temperature rise of said apparatus during said short time period to a desired maximum temperature, said means comprising a predetermined mass of material having a high latent heat of fusion, said heat of fusion being relatively large as compared to the specific heat of said apparatus, said material being disposed in a container which is inside said tank and in thermal absorbing relationship with respect to said apparatus, said material having a melting point temperature which is below said maximum temperature but above said low temperature range and said material being thermally reversible whereby said material will change from a solid to a liquid state during said short time period and then revert to said solid state during said low temperature range period.

3. In an electric transformer which has a generally repetitive thermal cycle which includes a period of operation at a relatively low temperature range and which may include a period of operation at a relatively high temperature range due to an overload, said high temperature range period being of relatively short time duration as compared to the total time duration of said cycle, and said transformer being housed in a main tank, means for limiting the temperature rise of said transformer during said short time period due to an overload to a predetermined maximum temperature, said means comprising a predetermined mass of naphthalene, said naphthalene being disposed in a container which is inside said tank and in thermal absorbing relationship with respect to said transformer, said naphthalene having a melting point temperature which is below said maximum temperature but above said low temperature range and said naphthalene being thermally reversible whereby said naphthalene will change from a solid to a liquid state during said short time period in the event of an overload and then revert to said solid state during said low temperature range period.

4. In an electric transformer comprising a magnetic core structure and linked electrical windings immersed in an insulating fluid within a sealed tank, means for controlling the temperature rise of said windings due to a short time overload by limiting the top insulating fluid temperature to a predetermined value, said means comprising a predetermined mass of material having a high latent heat of fusion, said material being disposed within a closed container and said container being positioned within the top insulating fluid, said latent heat of fusion being relatively large in comparison to the specific heat of said insulating fluid, said material having a predetermined melting point temperature whereby said material will change from a solid to a liquid state only during said short time overload by absorbing heat from said 8 top insulating fluid, and said material being thermally reversible.

5. In an electric transformer comprising a magnetic core structure and linked electrical windings, means for controlling the temperature rise of said windings due to a short time overload by limiting the core structure temperature to a predetermined value, said means com prising a predetermined mass of material having a high latent heat of fusion, said material being disposed in a closed container and said container being in direct contact with said core structure, said latent heat of fusion being relatively large in comparison to the specific heat of said core structure, said material having a predetermined melting point temperature whereby said material will change from a solid to a liquid state only during said short time overload by absorbing heat from said core structure, and said material being thermally reversible.

6. In an electric transformer comprising a magnetic core structure and linked electrical windings, means for controlling the temperature rise of said windings due to a short time overload, said means comprising a predetermined mass of material having a high latent heat of fusion, said material being positioned in a closed container which is positioned between the core structure and windings, said latent heat of fusion being relatively large in comparison to the specific heat of said core structure and windings, said material having a predetermined melting point temperature whereby said material will change from a solid to a liquid state only during said short time overload by absorbing heat from said core structure and windings, and said material being thermally reversible.

References Cited in the file of this patent UNITED STATES PATENTS 

